1. Field of the Invention
The present invention generally relates to the field of belt or chain tensioning roller devices, particularly automatic tensioning rollers, used in particular to ensure appropriate belt or chain tension, for example in timing belts or belts used to drive auxiliaries or the like. Automatic tensioning rollers such as these can be used in motor vehicle combustion engines and allow the belt tension to be kept always within a predetermined range.
2. Description of the Relevant Art
Tensioning rollers generally include a support intended to be fixed to an engine block, and a moving part that can move angularly with respect to the support. The moving part is provided with an eccentric and with a rolling bearing on which there is mounted a roller intended to be in contact with the belt. A spring constantly exerts a tensioning force between the support and the moving part, causing the roller to be in contact with the belt under appropriate tension. The moving part is mounted on the support with the possibility of adjustment so that the belt tension can be adapted. Thus, there are automatic tensioning rollers that include an adjustment eccentric used to preposition the roller against the belt, the eccentric then being immobilized with respect to the engine block using a screw, and a working eccentric mounted directly or indirectly about the adjusting eccentric and used to convert the torsional moment of a spring into a radial force on the belt. In operation, the working eccentric can oscillate angularly with respect to the adjustment eccentric or with respect to an intermediate part, thanks to bearing bushes that form a plain bearing. Reference may be made to document EP B 456 142.
Document FR 2 624 577 discloses a tensioning roller including a swinging arm pivotably mounted on a pivot pin fixed to the engine block. The arm is angularly actuated by a return spring in a direction that tends to press the roller against a belt. While such devices are satisfactory from the performance standpoint, their very high number of component parts is a disadvantage, and they are generally expensive to manufacture.
Document U.S. Pat. No. 6,129,643 describes a variable-diameter pulley including a part in contact with the belt that can off-center itself with respect to a part secured to the shaft. However, this type of pulley is heavy, wears quickly, carries a risk of vibration and noise, and is expensive, as much because of the number of component parts as because of their complexity, which parts need to be fitted together particularly accurately. The part in contact with the belt is in the form of a rigid annulus.
Document U.S. Pat. No. 7,033,293 describes an expanding pulley including a plurality of blocks separated from one another and in contact with the belt. The blocks can move radially between two sides of the pulley under the action of a conical element moved by a spring. Once again, the number of component parts is very high. Manufacturing and assembling the blocks are expensive. There is a risk that the conical component will become skewed, causing the pulley to become unbalanced and generating vibrations. Furthermore, the discontinuous contact between the blocks and the belt also causes vibrations and variations in the belt tension, which are likely to result in premature wear.